Considering the importance of mobile sources in the release of air pollutants in the city, the need to develop and use public transportation is very important, for this purpose there are different approaches. Karaj city needs to replace the fleet with updated standards due to the outdated public transport fleet. Considering the economic and environmental dimensions, the effectiveness of the measures taken should be investigated. Therefore, evaluating the change in the release of pollutants based on each alternative scenario can help the authorities to adopt air pollution reduction strategies. The purpose of designing scenarios in this research is to estimate the effectiveness of scenarios in the field of replacing worn-out taxi fleets in reducing the emission of air pollutants. First, the classification of taxis based on parameters such as different systems, production year, emission standard, type of fuel consumed and vehicle class have been investigated. In the following, the scenarios of standard pollutant reduction were designed in two plans, the replacement of the worn-out fleet with a hybrid, dual-fuel fleet with consumable fuel, with the Euro 4 pollution standard, and finally, the scenarios were compared with the base scenario. The emission coefficients and design scenarios were calculated using the International Vehicle Emission Model (IVE) for roads, 1st grade arterials, highways and freeways, with slopes of zero and ±2%. The results of the comparison of the scenarios showed that by replacing the entire fleet worn out in terms of age (to a new fleet, the second scenario), the highest reduction in standard pollutant emissions was 68% (CO, 85% (VOC), 53% (NOX), 100% ( SOX) and 67% (PM). Also, the results of pollutant emission distribution in Karaj city, using Arc Map software, showed that the highest and lowest pollutant emissions are related to zone 10 and zone 1, respectively. Scenarios for reducing pollutant emissions And the solutions to increase air quality are useful and largely operational for politicians and researchers to better understand the current conditions of air pollution in the region, which require timely provision of funds and principled and detailed planning for implementation.